I fitted spacers to the front fine but the same spacers don't seem to go on the rear. It's almost like the inner 'pockets' of the spacer aren't machined out enough meaning that it doesn't sit flush on the inner hub part. I hope I've explained that well enough.

Does anyone know of any 15mm spacers that will fit as I don't want to take a chance again and get them wrong. Also, has anyone else had this issue?

try the front spacers on the rear??

They will be snagging on the slight taper at the rear of the hub wheel bearing section. As above^ or might need to get them machined out a mm or two.

I think I explained it in one of your other threads or at least have many times before. Normally anything below 20mm won't fit because of the depth of the centre spigot that the wheel sits on. However I have fitted 15mm spacers from H+R that fitted fine, they were supplied by DPM performance IIRC.